Code-division multiple access (CDMA) is a method of multiplexing multiple channels onto the same spectral region. CDMA applies different codes to the different channels to allow them to be separated at the receiver. CDMA is commonly used in radio frequency (RF) communications. CDMA in the optical regime (Optical-CDMA or OCDMA) can also be performed. It has some attractive features, most notably an element of physical security since it can be difficult to measure the desired signal without knowing the correct code, and only the legitimate users possess the code. This is different from having wavelength division multiplexed channels which are easily separated using optical filtering technology. However, the security feature of OCDMA is often quite weak unless it is designed properly, and such a secure design may not be practical to implement. For instance, the code space is often very small allowing an eavesdropper (Eve) to simply try all codes until the desired channel becomes visible. Each channel generally uses a different code, and the equipment required to apply a code can be expensive. Also, to reduce inter-channel interference OCDMA often requires additional equipment such as high speed optical time gates which can make the method expensive to implement. Nevertheless, the potential of OCDMA for network security as well as other networking benefits including simplified bandwidth provisioning have attracted interest in the field.
One method of applying optical codes is to use a spectral phase encoder (SPE), such as in US 2006/0171722 A1, where an optical pulse is broken up into its constituent spectral components and each spectral component is phase shifted by the SPE, although other types of encoders can also be used. The phase shift applied at each spectral component is the code. SPEs typically have a fairly small code space, which is not good for maintaining high security levels. However, that limitation can be mitigated by using a dynamically varying code as in “Running-code optical CDMA at 2×10 Gbit/s and 40 Gbit/s,” by S. X. Wang et al in Electronics Letters, v46, Is 10 pp 701-702, 13 May 2010 and U.S. Pat. No. 7,831,049 B1. The dynamic code can be based off a short secret key seeding a pseudo-random number generator, thereby generating pseudo-random codes which vary in time. While using a dynamic code can make the scheme more secure, each channel still needs to be coded separately, therefore requiring many SPE elements when used in an optical network with multiple channels. Additionally, the codes are not orthogonal (without further effort) so there is channel-to-channel interference. A SPE can in principle be built in an integrated optical circuit, which could make the need for multiple units acceptable. However such an implementation of an encoder typically has a small code space and usually the code cannot be changed on a fast time scale. An acousto-optical modulator SPE can change codes much faster but is bulky.
A time-mode method of implementing SPE is possible such as described by X. Wang and N. Wada in “Spectral phase encoding of ultra-short optical pulse in time domain for OCDMA application,” in Optics Express v. 15, no. 12, Jun. 11, 2007. Here the individual pulses are spread out in a dispersive element to create chirped pulses. Chirped pulses have a spectral frequency which varies in time over the duration of the pulse, and therefore a standard temporally-modulated electro-optic phase modulator can apply a spectral code. As implemented by Wang and Wada this method is not easy to scale to high data rates since a series of repetitive time-mode phase shifts (codes) are applied to each pulse individually. In order to apply the same code to each pulse, the pulses cannot overlap in time after dispersion. The data rate of this method is thus limited by the temporal response of the modulator. For instance if 16 different phase chips are applied to each pulse, then the resulting single channel data rate will be 1/16th of the update rate of the phase modulator. In order to have a long code-length, which can enhance security and spectral efficiency, the data rate per channel would have to be quite low. Other methods of implementing an OCDMA system include using fiber Bragg gratings such as in U.S. Pat. No. 6,628,864 B2, which can have long code lengths, but cannot be quickly reprogrammed, if at all.
The frequency and thus phase of a laser can be modulated by changing the current through a semiconductor laser. Frequency is the time-derivative of phase, and thus frequency and phase shifts are related, however frequency shifts typically imply a large phase shift of >>2π occurring over a fixed time duration while a phase shift can be a small discrete phase shift<2π which is fixed over a time duration. The ability to change the frequency of a laser has been used to create phase shift keyed signals by changing the current for short intervals between bits to cause an associated phase shift such as in U.S. Pat. No. 5,050,176. It is relatively easy to create phase or frequency variations in this manner, though the magnitude of the frequency variation is limited by the fact that changing the laser current also changes the output optical power level. There are other ways to change the frequency of a laser such as the use of an external cavity with frequency selective feedback.
A parametric amplifier can be used to cause a shift in the optical center frequency (or equivalently its wavelength) of an optical signal, such as in U.S. Pat. No. 6,330,104 B1. These devices shift the wavelength of input signal light to an idler wavelength, where the idler wavelength depends on the signal wavelength and the pump wavelength used to pump the amplifier. For instance, in a typical fiber parametric amplifier fi=2fp−fs, where fi is the output idler optical frequency, fp is the pump optical frequency, and fs is the signal optical frequency. The optical frequency is related to optical wavelength (λ) by f=c/λ, where c is the speed of light so optical frequency and optical wavelength are directly related and either may be used to describe the same effect and translated between each other via the aforementioned equation.
What is needed is mechanism to encode and decode optical signals that can be dynamically varied quickly in time. It should make the resulting coded signal difficult to measure or manipulate for an eavesdropper, but be practical to implement for the legitimate users. If the optical coder could work on multiple wavelengths simultaneously its cost per wavelength would be reduced, which would be a substantial advantage. Additionally by operating on multiple wavelengths the power consumption per transmitted data bit and size requirements can also be reduced. Additionally, it is advantageous if the multiple wavelengths maintain their orthogonality or near orthogonality (no interference, or at least low levels of interference) after being decoded. It is also a benefit if the data modulation of the various wavelength channels to be encoded do not need to be temporally synchronized, possibly even operating at different data rates with different modulation formats. Thus the frequency encoding/decoding process does not need to be synchronized with the input data rates. It is also a benefit if the code could shift the frequency of the input signal over a large range, as this can make it more difficult for an eavesdropper to measure or manipulate the optical signal transmitted since the optical center frequency is shifted in time over a large bandwidth and the shifted frequency can overlap with other wavelength-division-multiplexed (WDM) channels.